The field of outdoor power equipment comprises a wide variety of units that perform various types of work operations in an outdoor environment. A common characteristic of such units is that they carry a prime mover of some type thereon that powers a traction drive system. In turn, the traction drive system powers one or more ground engaging traction drive members, such as wheels or endless tracks, which cause the unit to be self-propelled over the ground.
The traction drive systems of outdoor power equipment units often comprise one or more hydraulic pumps that are driven by the prime mover. The pressurized hydraulic fluid from such pumps is supplied to one or more hydraulic motors that rotate the traction drive members. The hydraulic pumps are usually variable displacement pumps whose output is infinitely variable from zero pump displacement to maximum pump displacement. This allows the operator to smoothly control the ground speed of the unit by infinitely controlling the pump output through a user operated ground speed control such as a foot operated traction pedal. The traction pedal can be either mechanically linked to the traction drive pump(s) or electronically linked through a fly-by-wire control system that employs an electronic controller for that purpose.
In addition to the above-described hydraulic traction drive system, outdoor power equipment units are equipped with at least one work implement for performing some type of work operation. For example, when the unit is a mower, the unit will be equipped with one or more grass cutting units or mower decks for cutting grass. When the unit is a trencher, the unit will be equipped with an endless trenching chain or circular cutting disc or wheel for cutting a trench in the ground. Such work implements are also ultimately driven by the same prime mover carried on the unit that powers the traction drive system. Like the traction drive system, the work implements are often hydraulically driven from the prime mover using at least one additional hydraulic pump connected to the prime mover for supplying pressurized hydraulic fluid to one or more additional hydraulic motors that rotate the work implements.
One difficulty experienced in the operation of outdoor power equipment units of this type is that the load experienced by the work implements can vary widely over a short period of time, thus causing the load demands on the engine to rise or fall quickly as well. For example, if the unit comprises a mower, the mower may at one moment be cutting grass that is relatively short and sparse and then at the next moment move into an area where the grass is much longer and thicker than before. When this happens, the available engine horsepower may be insufficient to both maintain the vehicle speed and to cut the longer and thicker grass properly.
If the operator tries to maintain the mower ground speed at the same speed when cutting longer and thicker grass as when cutting shorter and sparser grass, then the quality of cut being provided by the mower will decline. In other words, the grass will not be cut as evenly and uniformly as before at and after the point at which the available engine horsepower is fully utilized. When encountering this situation, the operator can alternatively listen for the decrease in engine speed, known as engine lug, which accompanies the situation where the load requirements on the engine from the traction drive and the work implements exceed the available horsepower. When the operator hears or otherwise detects from the performance of the mower that the engine is lugging down, the operator can then manually ease up on the traction pedal to slow the mower down to decrease the traction drive load in an attempt to maintain the quality of cut. However, this latter approach requires a very skilled operator who is constantly attentive to engine lug to be even somewhat successful. Even with a skilled and attentive operator, the inevitable slow response time of a human being means that the available engine horsepower is still often exceeded when the instantaneous traction drive and work implement loads are greater than that horsepower—the operator tends to react too slowly.
The situation is even worse when a less experienced or skilled operator is operating the mower. In situations where the available engine horsepower is less than the load requirements and the engine begins to lug down, the natural reaction of most inexperienced operators is to push further down on the traction pedal as they would in an automobile or car whose engine exhibits the same behavior. However, in an outdoor power equipment unit, this only exacerbates the situation by increasing the load imposed by the traction drive system. The result is an engine stall and a complete stoppage of the mower.
In an attempt to assist the operator to better utilize the available engine horsepower, various control systems and methods have been proposed that automatically control the operation of the traction drive system in response to a decline in engine speed below a predetermined reference engine speed. For example, U.S. Pat. No. 6,675,577 to Evans discloses such a system for use on vehicles for industrial and agricultural use, such as compact utility tractors. In Evans, the speed of the engine is sensed by a sensor. When the speed of the engine drops below a target value to thereby indicate the beginning of excessive engine lug caused by load, the control system disclosed in Evans automatically reduces the stroke of a hydraulic pump that powers the traction drive system to automatically reduce the vehicle speed and prevent engine stall. Consequently, this relieves the operator from having to manually detect engine lug and to then manually ease off the traction pedal in an attempt to do the same thing.
A control system such as that disclosed in Evans has advantages. It assists all operators, but particularly less skilled and inexperienced operators, in the efficient operation of the outdoor power equipment unit by preventing the instantaneous load requirements on the engine from exceeding the available engine horsepower. Thus should greatly reduce, though probably not entirely eliminate, most engine stalls. In addition, it should enhance productivity through higher average horsepower utilization during the operation of the outdoor power equipment unit.
However, a system as disclosed in Evans is incapable of addressing every load situation experienced during the operation of the outdoor power equipment unit. There will be times when the load imposed by the work implements may be so extreme that it would require slowing the ground speed too much, thereby creating an unsafe operational condition. Moreover, a system as in Evans ignores the fact that many outdoor power equipment units have other load drains on the engine that arise from the operation of other components besides the traction drive system and the work implement. Accordingly, a control system that would also take into account these issues would be a further advance in the art.